This invention relates to liquid metal cooled fast breeder nuclear reactors.
In a liquid metal cooled fast breeder nuclear reactor the fuel assembly, and sometimes a fuel store for irradiated and new fuel, is supported on a diagrid in a containing vessel and the diagrid serves as a plenum for distributing coolant throughout the fuel assembly. Because of the weight of the fuel assembly, several hundred tonnes, it is necessary to provide strengthening structure such as webs in the plenum but such strengthening structure besides making manufacture of the diagrid more complicated, also sets up complex stresses in the plenum which could give rise to a breach with consequent loss of coolant. As a protection against such an emergency it has been proposed to make the plenum of double containment construction whereby in the event of failure of the first containment the second containment, which need not necessarily be leak tight, will contain the pressure long enough for an orderly shut down of the reactor to be effected. Double containment construction leads to greater complexity and it is an object of the invention to provide an improved liquid metal cooled fast breeder nuclear reactor having a diagrid of simplified construction which thereby reduces the risk of failure due to complex stresses but at the same time provides a secondary containment for reducing the rate of leakage from a breached first containment.